Method of reducing audio gap in downlink during handoff of cellular radiotelphone

ABSTRACT

A method of reducing an audio gap in a signal connection with a communication unit during handoff in a cellular communication system is provided. The method includes the steps of providing a temporary signal connection with the communication unit from a mobile switching center (MSC) of a source base site, through the source base site to a handoff target base site and establishing a trunk connection from the MSC of the source base site to a MSC of the target base site. The method further includes the step of interconnecting, at the target base site, the temporary signal connection, trunk connection, and communication unit; and re-routing the signal connection with the communication unit through the trunk connection.

This is a continuation-in-part of application Ser. No. 08/081,895, filedJun. 25, 1994, now U.S. Pat. No. 5,386,456.

FIELD OF THE INVENTION

The field of the invention relates to communication systems and inparticular to cellular communication systems.

BACKGROUND OF THE INVENTION

Cellular communication systems are known. Such systems are typicallyconstructed with a number of partially overlapping service; coverageareas (cells) within which communication services may be provided tocellular radiotelephones (also commonly referred to as a communicationunits or mobile stations (MSs)) passing through such cells.Communication services within a cell are provided by a base transceiverstation (BTS) typically located at a base site.

Cellular communication systems are typically organized with a number ofBTSs located at a base site and under the control of a single base sitecontroller (BSC). A BSC and associated BTSs are commonly referred to asa base station system (BSS). A number of BSSs are, in turn, controlledby a mobile switching center (MSC). The MSC in addition to routing callsamong BTSs also provides in interconnect with a local public switchtelephone network (PSTN) and to other MSCs.

A cellular system, covering a geographic area, allows an MS tocommunicate among other MSs or with public switch telephone network(PSTN) subscribers through the PSTN interconnect provided at the MSC.Calls received from an MS through a BTS are routed to the BSC which, inturn, routes the call request to the MSC. The MSC then routes the callto the desired party.

The BSC provides message transfer and call switching as directed by theMSC. Control information received from an MS through a BTS of the BSCare routed to the MSC. Calls received by a BSC from the MSC, on achannel of a communication link between the MSC and BSC, are switchedunder control of the BSC to the appropriate BTS in communication withthe MS. Routing of all calls (including those between MSs under the sameBSC) are routed through the MSC.

The BSC also typically provides a convenient location for transcodingvoice information from a pulse coded format (PCM) used within the PSTNto a compressed version exchanged over the air interface. Speechcompression may be accomplished within the transcoder using a linearpredictive coding algorithm.

The MSC upon receipt of a call from an MS first determines whether thecalled party (target) is a MS under its control (within the MSCsgeographic area). The MSC may make such a determination by reference toa call history stored within a memory of the MSC. If the target is a MSformerly within the MSC's geographic area, the MSC causes a page messageto be generated for the target which is then transferred to each BSCcovering the locale the MS was last known to be in. The BSCs then causethe page to be transmitted through each antenna of each BTS connected tothe BSC. Upon identification of a location of a target the MSC issuesswitching commands, directed to requesting and target BSCs, providing asignal path therebetween.

If the MSC determines the target to be a PSTN subscriber, or asubscriber of another MSC (covering a different geographic area), thenthe MSC composes a call request message to be transmitted into a propertrunk group of the PSTN interconnect or a proper trunk group interfacingMSCs. Trunk groups, in general and channels within trunk groups inspecific, are associated with specific geographic areas (and identity oftarget). An MSC determines the geographic area in which the target islocated (and identifies whether the target is a PSTN subscriber or MS)by a translation of the called number. Upon identification of a type oftarget and geographic area the MSC transmits the call request into thetrunk group associated with the target's identity and geographic area.

If the call request is received from the PSTN interconnect or from theMSC interconnect, then the MSC composes a page message for transmissionthrough each BSS within the system. If the MS is within the system theMS responds through the nearest BTS. The response is transferred to theBSC, which in turn transfers the response to the MSC along with anidentification of the receiving BTS. The MSC matches the call responsewith the call request and allocates a signal path from the interconnectto the BSC, along with instructions to the BSC to connect the selectedchannel of the receiving BTS to the allocated signal path.

Where a communication link has been established between a MS and target,and the MS moves from a first cell into a second cell (target cell), thecommunication link must be re-established through a BTS of the targetcell or the call will be lost. As an aid in identifying the target cell,MSs typically measure signal strengths of signals from surrounding BTSsand transmit signal readings (and identities of a measured BTS) to aserving (source) BTS. The source BTS, in turn, transfers the readings tothe MSC for evaluation. Where the target cell is served by the same MSCthe MSC simply re-routes the call to the target BTS.

Where the target BTS is served by a second MSC (target MSC), theoriginating MSC (source MSC) must establish a communication path acrossa seam in the cellular communication system. The originating MSC doesthis by requesting access to the target BTS through a trunk groupassociated with the target MSC. The time required to re-route the callthrough the trunk group and target MSC results in an interruption ofaudio information that is perceived by a user as an audio hole or gap.Because of the importance of cellular communication a need exists for ameans of re-routing calls between cells served by different MSCs thatdoes not create audio gaps.

SUMMARY OF THE INVENTION

A method of reducing an audio gap in a signal connection with acommunication unit during handoff in a cellular communication system isprovided. The method includes the steps of providing a temporary signalconnection with the communication unit from a mobile switching center(MSC) of a source base site, through the source base site to a handofftarget base site and establishing a trunk connection from the MSC of thesource base site to a MSC of the target base site. The method furtherincludes the step of interconnecting, at the target base site, thetemporary signal connection, trunk connection, and communication unit;and re-routing the signal connection with the communication unit throughthe trunk connection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 comprises a block diagram of a cellular communication system inaccordance with the invention.

FIG. 2 comprises a block diagram depicting the four stages of handoff inaccordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The solution to the problem of audio gaps in cellular handoff among MSCslies, conceptually, in creating a temporary inter base site signalconnection (peer-to-peer signal path) between source and handoff targetbase sites. The temporary signal connection is maintained until a trunkconnection is established between the MSC of the source base site(source MSC) and the MSC of the target base site (target MSC).

FIG. 1 is a block diagram of a cellular communication system 10 inaccordance with an embodiment of the invention. Included within thecommunication system 10 is a first MSC 12 providing communicationservices within a first area through a first and second base site (BSCs15-16 and BTSs 20-23). Also included is a second MSC 13 Providingcommunication services within a second area through a third and fourthbase site (BSCs 18-19 and BTSs 24-27). Handoff of calls between thefirst area and the second area, in a first instance, is accomplishedthrough MSC trunk group 14. Temporary call connections between the firstarea and second area, in the second instance, is accomplished throughvoice circuits 17 interconnecting BSCs 15-16 in the first area with BSCs18-19 in the second area.

By way of example a communication unit (MS 11) initiates a call to aPSTN subscriber (not shown). In support of the call the MS 11 transmitsan access request 30 to BTS 23. The BTS 23 transfers the request to theBSC 16. The BSC 16, in turn, transfers the access request to the MSC 12.In response to the access request, the MSC composes a call requestmessage for transmission into the appropriate trunk group of the PSTN.The MSC 12 also allocates a signal path to the BTS 23 and forwardsinstructions to the BSC 16 for allocation of a traffic channel at theBTS 23 for use by the MS 11. Upon completion of call set-up the MS 11begins communicating with the PSTN subscriber.

As the call continues the MS 11 moves into the cell of BTS 24. The MS 11detects the movement through signal measurements of nearby BTSs (e.g.,22-25). In due course the signal measurements are transferred to ahandoff source BTS 23 (source BTS) and in turn to the MSC 12 (sourceMSC). The source MSC 12 (or the BSC in some systems) based upon acomparison of the signal measurements with threshold values determinesthat BTS 24 is the best handoff target (target BTS).

Since the MSC 12 cannot predict the precise instant when the MS 11 willchange channels, a temporary signal connection 17 is established by thesource MSC 12 between a transcoder in the source BTS 16 and the targetBTS 24. The use of the temporary connection 17 allows for thesimultaneous transmission of signal information to the MS 11 through thehandoff target BTS 24 (on a new channel allocated by the target BTS 24)and transmission of signal information on the channel allocated in thesource BTS 23.

Since a BTS is capable of handling only a limited number of calls, thetemporary connection from the source BTS 23 to the target BTS 24 must bereplaced by a trunk connection through MSC trunk group 14. In support ofestablishing such a connection the source MSC 12 composes a calltransfer message for transmission through the proper trunk group of theMSC trunk group 14 to the MSC 13 of the target BTS 24 (target MSC).

The target MSC 13 in response to the message forwards instructions tothe BSC 18 of the target BTS 24. The instructions cause the BSC 18 toallocate a traffic channel to the MS 11 through the BTS 24. Theinstructions also cause the BSC 18 to provide a three-party connectionwithin the BSC 18 with a first and a second connection of thethree-party connection dedicated to the traffic channel of the BTS 24and the temporary signal connection 17 respectively. The thirdconnection of the three-party connection is dedicated to a signal paththat the target MSC 13 establishes between the BSC 18 and the source MSC12 (the MSC trunk connection).

The use of the three-party connection at the target BSC 18 allows forsignal information from either signal path (temporary signal connectionand MSC trunk connection) to be summed and transmitted to the MS 11through the target BTS 24. Summing and transmitting signal informationprovides the beneficial effect of reducing audio gaps when the instantof handoff cannot be determined.

Following transmission of a handoff command by the source MSC 12 throughthe source BTS 23, set-up of the temporary signal connection between thesource BSC 16 and target BSC 18, and channel allocation by the targetBTS 24, the system 10 waits for transmission of a handoff completemessage by the MS 11 on the channel allocated through the target BTS 24.Following receipt of the handoff complete message and set-up of the MSCtrunk connection the three-party connection is bypassed and thetemporary connection abandoned.

Shown in FIG. 2 are four block diagrams depicting the four steps ofhandoff in accordance with the invention. In a first step a temporarysignal connection 17 has been established in accordance with theinvention. An information signal transceived through the source MSC 12under pulse code modulation (PCM) is transferred from the MSC 12 to atranscoder (XCDR) 31 in the source BTS 16 and to a XCDR 32 in the targetBSC 18. Within the XCDRs 31 and 32 the PCM signal is converted into aproper format for transfer over the traffic channel.

In a second step of the handoff the target MSC 13 has established athree-party connection 33 within the target BSC 18. Downlink informationof a downlink signal stream (from PSTN subscriber to MS 11) istransferred from the source BSC 16 to the MS 11 through the source BTS23 and through the three-party connection 33 within the target BSC 18.Since the target MSC trunk connection has not yet been established thetarget MSC 13 is providing a quiet tone to the third connection of thethree-party connection 33, thereby having no effect on the signaltransmitted to the MS 11.

In the uplink direction (MS 11 to PSTN subscriber) the XCDR 32 istranscoding whatever traffic channel information is being received onthe allocated channel through the target BTS 24. The source BTS 23,likewise, is transcoding traffic channel information. The XCDR 31 withinthe source BSC 16 selects the PCM signal from the BTS (source BTS 23 ortarget BTS 24) on which a signal is detected from the MS 11.

In the third step, the MSC trunk connection (from the source MSC 12 tothe target BSC 18) has been completed and a handoff complete message hasbeen received from the MS 11. Following receipt of the handoff completemessage, a bypass 34 is established around the three-party connection33. Following bypass of the three-party connection 33, the three-partyconnection 33 and temporary signal path 17 are abandoned.

In another embodiment of the invention and during the first step ofhandoff, the temporary signal connection from source BSC 16 to targetBSC 18 is established through the source MSC 12 and target MSC 13 on atemporary trunk connection. During the second step a three-partyconnection is established all the target BSC 18 in the downlink amongthe target BSC 18, the downlink of the temporary trunk connection, andthe downlink of the permanent target MSC trunk connection. As above,downlink information of a downlink signal stream (from PSTN subscriberto MS 11) is transferred from the source BSC 16 to the MS 11 through thesource BTS 23, the temporary trunk connection and through thethree-party connection 33 within the target BSC 18. Since the permanenttarget MSC trunk connection has not yet been established the target MSC13 is providing a quiet tone to the third connection of the three-partyconnection 33, thereby having no effect on the signal transmitted to theMS 11.

In the third step, the permanent MSC trunk connection (from the sourceMSC 12 to the target BSC 18) has been completed and a handoff completemessage has been received from the MS 11. Following receipt of thehandoff complete message, a bypass is established around the three-partyconnection. Following bypass of the three-party connection 33, thethree-party connection and temporary signal path are abandoned.

In another embodiment of the invention the temporary signal connectionand three-party connection are used to reduce an audio gap duringhandoff among BSCs 15, 16 under the same MSC. The temporary connectionamong BSCs 15, 16 may be accomplished by a peer to peer connection orthrough a temporary connection through the MSC 12. As above thethree-party connection is used to sum audio from the temporaryconnection until the permanent connection becomes available. As above,when the permanent connection becomes available the temporary connection(and three-party connection) is abandoned.

The many features and advantages of this invention are apparent from thedetailed specification and thus it is intended by the appended claims tocover all such features and advantages of the system which fall withinthe true spirit and scope of the invention (e.g., a duplex temporarysignal connection where uplink and downlink signal processing areidentical). Further, since numerous modifications and changes willreadily occur to those skilled in the art (e.g., the transcoder may bein the MSC), it is not desired to limit the invention to the exactconstruction and operation illustrated and described, and accordinglyall suitable modifications and equivalents may be resorted to, fallingwithin the scope of the invention.

It is, of course, to be understood that the present invention is, by nomeans, limited to the specific showing in the drawing, but alsocomprises any modification within the scope of the appended claims.

We claim:
 1. A method of reducing an audio gap in a signal connectionwith a communication unit during handoff in a cellular communicationsystem, the method comprising the steps of:establishing a temporaryinter-base site signal connection between a source base site and atarget base site for routing signals from the communication unit throughthe target base site to the source base site; establishing a trunkconnection between a source mobile switching center associated with thesource base site and a target mobile switching center associated withthe target base site; interconnecting, at the target base site, thetemporary inter-base site signal connection, the trunk connection andthe communication unit; and routing signals from the communication unitto the source mobile switching center through the trunk connection. 2.The method as in claim 1 wherein the step of establishing a temporaryinter-base site signal connection comprises establishing a signal pathbetween a transcoder of the source base site and a transcoder of thetarget base site.
 3. The method as in claim 1 wherein the step ofinterconnecting the temporary inter-base site signal connection, thetrunk connection and the communication unit comprises establishing athree party connection by linking the temporary signal connection, adownlink from the trunk connection and a traffic channel assigned to thecommunication unit by the target base site.
 4. The method as in claim 3further comprising the steps of:receiving from the communication unit ahandoff complete message; establishing a signal connection from thetarget mobile switching center and the target base site bypassing thethree party connection; and terminating the temporary inter-base sitesignal connection and the three party connection.
 5. A method ofreducing an audio gap in a signal connection with a communication unitduring handoff in a cellular communication system, the method comprisingthe steps of:establishing a temporary inter-base site signal connectionbetween a source base site and a target base site via a temporary trunkconnection between a source mobile switching center associated with thesource base site and a target mobile switching center associated withthe target base site for routing signals from the communication unitthrough the target base site to the source base site; establishing apermanent trunk connection between the source mobile switching centerand the target mobile switching center; interconnecting, at the targetbase site, the temporary trunk connection, the permanent trunkconnection and the communication unit; and routing signals from thecommunication unit to the source mobile switching center through thepermanent trunk connection.
 6. The method as in claim 5 wherein the stepof interconnecting the temporary trunk connection, the permanent trunkconnection and the communication unit comprises establishing a threeparty connection by linking a downlink from the temporary trunkconnection, a downlink from the permanent trunk connection and a trafficchannel assigned to the communication unit by the target base site. 7.The method as in claim 6 further comprising the steps of:receiving fromthe communication unit a handoff complete message; establishing a signalconnection from the target mobile switching center and the target basesite bypassing the three party connection; and terminating the temporaryinter-base site signal connection and the three party connection.
 8. Amethod of reducing an audio gap in a signal connection with acommunication unit during handoff in a cellular communication system,the method comprising the steps of:establishing a temporary inter-basesite signal connection between a source base site and a target base sitefor routing signals from the communication unit through the target basesite to the source base site; establishing a permanent signal connectionbetween a mobile switching center and the target base site;interconnecting, at the target base site, the temporary inter-base sitesignal connection, the permanent signal connection and the communicationunit; and routing signals from the communication unit to the mobileswitching center through the permanent connection.
 9. The method as inclaim 8 wherein the step of interconnecting the temporary inter-basesite signal connection, the permanent signal connection and thecommunication unit comprises establishing a three party connection bylinking the temporary inter-base site signal connection, the permanentsignal connection and a traffic channel assigned to the communicationunit by the target base site.
 10. The method as in claim 9 furthercomprising the steps of:receiving from the communication unit a handoffcomplete message; and terminating the temporary inter-base site signalconnection and the three party connection.